A pretty cool iPhone 4 accessory comes in the form of this self-adhesive laser cut heat sensitive back. Made by RF Laserworks, it will change colors in the range 90 to 95 degrees F – or just below body temperature. The heat sensitive backing’s colors range from black (cold), all the way through red, yellow, and green before arriving at a deep blue (hot). It’s really thin measuring at only .005″ thick, so it can fit nicely under a bumper/case when not in use.